When I went to investigate this, I tested how a lot of user agents currently handle their numbering rules, and none of them match the current spec. (and very few come close to the spec. or each other, and no major ones match each other exactly).
- Lynx comes the closest to what the HTML5 spec. currently says, but is the only browser that performs incorrectly on the (invalid) initial positive sign symbol (U+002B PLUS SIGN).
- Only Opera and Lynx support negative numbers at all (except through unhandled overflows).
- IE and Webkit do not support the value 0.
- An empty string is treated according to the spec. by Gecko, Webkit, Presto and Lynx. It always returns 0 for w3m and Dillo. It always returns 1 for IE.
- There's a three-way split between how invalid values that begin with valid characters, are currently interpreted. Gecko ignores the entire value, treating it as if no value was specified (incrementing the previous value by 1). IE ignores the whole value but returns an absolute value of 1. The rest, per the spec., read all valid characters up to the first invalid character, and use that.
- As a special case of the above, all browsers will use a value that is invalid, due to containing a whitespace character as the first invalid character (that is, contains valid digits and then one or more whitespace characters and then any other non-whitespace characters), using the characters up to the first invalid character as above.
- As a special exception of the above, Gecko, if the first invalid character is a period (U+002E FULL STOP) but there are valid numeric digit characters after it, will use those digits as a number, ignoring the period.
- Presto, w3m and Dillo will treat invalid values as a 0. IE will treat invalid values as a 1. All others treat invalid values as an increment of the previous value.
Skip table; skip to Details.
Sample Ordered List Value Attributes
| Markup | HTML5 | IE | Gecko | Webkit | Presto | Lynx | w3m | Dillo |
|---|
| <li ></li> | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| <li value="0" ></li> | 0 | 2 | 0 | 2 | 0 | 0 | 0 | 0 |
| <li value="1" ></li> | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| <li value="3a" ></li> | 3 | 1 | 3 | 3 | 3 | 3 | 3 | 3 |
| <li value="3b" ></li> | 3 | 1 | 4 | 3 | 3 | 3 | 3 | 3 |
| <li value="2 " ></li> | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |
| <li value="2 1"></li> | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |
| <li value="2.1"></li> | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |
| <li value=".2" ></li> | 3 | 3 | 2 | 3 | 0 | 3 | 0 | 0 |
| <li value="-3" ></li> | −3 | 3 | 0 | 4 | −3 | −3 | 0 | 0 |
| <li value="+3" ></li> | 3 | 3 | 3 | 3 | 3 | −2 | 3 | 3 |
| <li value="-3" ></li> | −3 | 4 | 0 | 4 | −3 | −3 | 0 | 0 |
| <li value="" ></li> | −2 | 1 | 1 | 5 | 0 | −2 | 0 | 0 |
| <li ></li> | −1 | 2 | 2 | 6 | 1 | −1 | 1 | 1 |
| <li value="" ></li> | 0 | 1 | 3 | 7 | 0 | 0 | 0 | 0 |
| <li value="4" ></li> | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 |
| <li value="5" ></li> | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 |
| <li ></li> | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 |
| <li value="c" ></li> | 7 | 1 | 7 | 7 | 0 | 7 | 0 | 0 |
IE
- By default, the value is an automatic increment of the previous number
- Values greater than 2147483647 return 2147483647
- Values less than 0 return an automatic increment of the previous number
- An empty string returns 1
- A string not beginning with any numeric characters (including U+002B PLUS SIGN, U+002D HYPHEN-MINUS) returns 1
- Note: automatic increments of the previous value are 1 if there is no previous value, but overflow to -2147483648 if the previous value was 2147483647
Gecko
- By default, the value is an automatic increment of the previous number
- Values greater than 2147483647 return an automatic increment of the previous number
- Values less than 0 return 0, unless it's also less than -2147483647, in which case it returns an automatic increment of the previous number
- An empty string returns an automatic increment of the previous number
- A string not beginning with any numeric characters (including U+002B PLUS SIGN, U+002D HYPHEN-MINUS) returns an automatic increment of the previous number
- Note: strings beginning with U+002E FULL STOP ignore that character
- Note: automatic increments of the previous value are 1 if there is no previous value, but overflow to -2147483648 if the previous value was 2147483647
Webkit
- By default, the value is an automatic increment of the previous number
- Values greater than 2147483647 return an automatic increment of the previous number
- Values less than 0 return an automatic increment of the previous number
- An empty string returns an automatic increment of the previous number
- A string not beginning with any numeric characters (including U+002B PLUS SIGN, U+002D HYPHEN-MINUS) returns an automatic increment of the previous number
- Note: automatic increments of the previous value are 1 if there is no previous value, but overflow to -2147483648 if the previous value was 2147483647
Presto
- By default, the value is an automatic increment of the previous number
- Values greater than 536870911 return 536870911
- Values less than -536870912 return -536870912
- An empty string returns 0
- A string not beginning with any numeric characters (including U+002B PLUS SIGN, U+002D HYPHEN-MINUS) returns 0
- Note: automatic increments of the previous value are 1 if there is no previous value
Lynx
- By default, the value is an automatic increment of the previous number
- Values greater than 2147483647 return 2147483647
- Values less than -29997 return -29997
- Note: This behavior is documented: http://lynx.isc.org/current/lynx2-8-8/lynx_help/Lynx_users_guide.html#Lists
- An empty string returns an automatic increment of the previous number
- A string not beginning with any numeric characters (including U+002D HYPHEN-MINUS) returns an automatic increment of the previous number
- Note: automatic increments of the previous value are 1 if there is no previous value
w3m
- By default, the value is an automatic increment of the previous number
- Values greater than 2147483647 return 2147483647
- Values less than 0 return 0
- An empty string returns 0
- A string not beginning with any numeric characters (including U+002B PLUS SIGN, U+002D HYPHEN-MINUS) returns 0
- Note: automatic increments of the previous value are 1 if there is no previous value
Dillo
- By default, the value is an automatic increment of the previous number
- Values greater than 2147483647 return 2147483647
- Values less than 0 return 0
- An empty string returns 0
- A string not beginning with any numeric characters (including U+002B PLUS SIGN, U+002D HYPHEN-MINUS) returns 0
- Note: automatic increments of the previous value are 1 if there is no previous value
- Versions of user agents included the latest versions, all commonly-used versions, and many others, including extremely old versions (for example, IE was tested between versions 3 and 9). Occasional variations exist only in extremely old versions (for example, IE version 2 has different behavior to all other versions), however, no other browsers, or old versions of browsers, seem to be worth reporting on due to the small number of active deployments.
- The value 2147483647 is the highest value that can be stored in a signed 32-bit integer. The value -2147483648 is the lowest value that can be stored in a signed 32-bit integer. In principle, software compiled using a different int width may have different, corresponding limits; although in practice this is very rare.
- Also, note that the current HTML5 algorithms is slightly inconsistent with CSS (which explicitly allows U+002B PLUS SIGN to indicate positive integers). I don't know if mapping this attribute to CSS is a concern.
Personally, I don't have a solution to propose; this is purely informative.